Investigations have been conducted on five-membered heterocyclic polymers which have high stability in atmosphere and high electric conductivity. For example, U.S. Pat. No. 3,574,072 discloses polymerization of five-membered electroconductive polymers by electro-oxidation in a polar organic solvent containing water (water content: below 25%). However, favorable films are not obtained in an organic solvent containing no water.
In addition, a literature reference (Mol. Cryst. Liq. Cryst., 83, 1297, 1983) discloses a process for preparing thick films, of which electric conductivity is 0.01 to 100 S/cm, in an organic solvent with various anions such as tetrafluoroborate, hexafluorophosphate, perchlorate, hydrogen sulfate, fluorosulfonate, p-toluenesulfonate and trifluoroacetate. This process has disadvantages in that physical properties of electroconductive polymers are greatly influenced by anions, which eventually render difficulties in utilizing them because of brittleness (elongation: 2 to 3%).
Consequently, a process to synthesize electroconductive polymers in aqueous solution with a polymeric electrolyte (J. Chem. Soc., Chem. Commun., P. 327, 1987) or sodium alkyl sulfate (C.sub.n H.sub.2n+1 OSO.sub.3 Na, wherein n is an integer of 4 to 16) as an electrolyte has been attempted to solve such a problem. However, the polymeric anion remarkably increases elongation up to 400%) of the electroconductive polymer, providing a flexible film, while it reduces electric conductivity to 10.sup.-3 S/cm. Otherwise, a sulfate ion having a long alkyl group as a dopant provides a good film of excellent physical properties as well as electric conductivity ranging from 5 to 160 S/cm.
A literature reference (Synth. Met., 20, p. 119, 1987) states that electro-polymerization in an aqueous solution causes an irreversible reaction between strong nucleophilic water and a cationized electroconductive polymer and gives the polymer poor electrochemical reversibility caused by electrochemically inactive ketone and hydroxy groups formed in the ma-n chain of the polymer.